This invention relates to a process and apparatus for proportioning a plurality of liquid components to form a multicomponent liquid mixture.
When administering medical grade solutions to a patient or when packaging medical grade solutions, it is necessary to proportion accurately the individual components used to form the final solution composition. In addition, it is necessary to control the volume of the solution being administered or packaged for subsequent administration to a patient. Thus, it is necessary to control the volume of each constituent forming the final solution over the entire time period of administration or packaging. It is common practice to measure the volume of a liquid in a container by measuring the pressure at the bottom of the liquid column of known density wherein the container has a known cross section over the entire column height according to the equation: EQU P=.rho. g h EQU and V=A h
wherein P is the pressure at the bottom of the liquid column in the container due to the liquid column, .rho. is the liquid density, g is acceleration due to gravity, h is the liquid column height A is the container cross sectional area, which may be a function representing cross sectional area as a function of height, and V is the liquid volume in the container.
Unfortunately, presently available pressure sensing devices such as pressure sensing transducers, are prone to drift over time so that the indicated pressure indicated by the device is outside the allowable range of error and the measurements become unreliable.
It has been proposed in U.S. Pat. No. 4,597,507 to provide apparatus for metering a premixed solution to a container having a pressure sensing device which provides an output signal proportional to the solution volume in the container. Once the desired volume has been determined, it is then removed from the container and directed to the point of use. The pressure output signal is used to activate or inactivate a liquid inlet valve and a liquid outlet valve associated with conduits connected to the container. An overflow outlet is provided with the container so that it can be periodically filled completely and the pressure measured. This pressure measurement together with the known container volume is used to determine the density of the liquid within the container. This density measurement is effected independently of the container's normal use so that normal container use must be interrupted to obtain the measurement. No means are provided for compensating for pressure sensor drift. Accordingly, the apparatus provides no means for determining the integrity of the pressure sensor measurements over time and, due to the inevitable pressure measurement drift with the apparatus in this art, volume measurements will become inaccurate. In medical applications, inaccurate volume measurements cannot be tolerated.
Accordingly, it would be desirable to provide an apparatus which is capable of accurately proportioning individual liquid components to form a final desired liquid composition. In addition, it would be desirable to provide such an apparatus which maintains the desired measurement accuracy over long time periods.